T-Mobile does have a limited 3G network in the US, but to my understanding it works on a different frequency than what other carriers (including AT&T) use. I'm not sure if you're asking if you will ever be able to use the Bold on T-Mo's 3G network, but as of right now I believe the answer would be no due to the difference in frequencies.

__________________"You ever make fun of someone so much, you think you should thank them for all the good times you've had?"

Tmo running on 1700 MHz... and i think Att are running on 1900 MHz... correct me if i am wrong...
BOld support 850 MHz, 1900 MHz, and 2100 MHz....
G1 Support 1700 MHz, and 2100 MHz...
so if TMO... can run on 2100Mhz... than we are in luck..!!!